The invention relates to treatment of wastewater from residential, business, industrial, or other sources.
Wastewaters obtained from various sources can contain bulk solid materials (sticks, rags, sand, and other materials that can be screened or filtered therefrom), smaller suspended solid materials that cannot be easily removed by screening or filtration, degradable organic compounds (sometimes designated biological oxygen demand or “BOD”), pathogenic microorganisms (e.g., bacteria, viruses, spores, cysts, and protists), and undesirable nutrients (e.g., nitrogen- or phosphorous-containing compounds that can encourage nuisance growth of algae or weeds at the wastewater discharge site). Discharge of these materials into the environment with the wastewater can have undesirable health and environmental consequences. As a result, wastewater discharges are closely regulated by state and national governments.
Many methods exist for removing undesirable components from wastewater prior to its discharge. Typically, a wastewater treatment method might include one or more treatment steps in which settleable and floatable solids materials are removed, one or more (aerobic and/or anaerobic) microbial degradation steps in which most suspended solids and BOD are removed or captured in biological solids or slimes produced in the degradation step, and one or more further treatment steps in which wastewater is clarified, disinfected, or further purified.
Clarifiers are devices that are used to remove suspended solids (including biological solids) from wastewater streams. In a clarifier, solids are allowed to settle from a wastewater stream by maintaining wastewater substantially still in a vessel, so that denser solids can separate by the action of gravity from less dense liquids. Clarifiers (usually designated “primary” clarifiers) are often used at an early phase of wastewater treatment to remove relatively easily-segregated solids prior to microbial degradation (i.e., “digestion”) of less easily removed solids and BOD. Clarifiers (sometimes designated “secondary clarifiers”) are also used to remove microbial biomass and other solids (e.g., fine solids captured on or in biomass) from wastewater following digestion processes. Because water treated in the secondary clarifier is often discharged into the environment, the clarifying function of the secondary clarifier is generally more important than the same function of the primary clarifier.
In a conventional clarifier, wastewater is fed to a central region of a large pool. Often, the central region is physically segregated from the peripheral regions of the pool, although liquid can flow from the central region to the other regions of the pool. The physically segregated region is sometimes referred to as a stilling well, since the barrier that defines this chamber is intended to reduce transfer of kinetic energy from the wastewater flow entering the unit, in order to reduce turbulence in peripheral areas of the pool (i.e., stilling the water in the pool). The chamber is in fluid communication with the peripheral areas of the pool (normally by way of a passage at the bottom of the stilling well) and permits gentle mixing between liquid in the pool and newly added wastewater. Solids that enter the clarifier by way of the stilling well settle to the floor of the clarifier by the action of gravity in the still liquid in the clarifier, and accumulate on the floor as sludge.
In activated sludge-based digestion processes, biological solids removed from a post-digestion processing stream (e.g., sludge removed from the bottom of a clarifier) can be recycled into the biological reactor in order to enhance the speed or efficiency of the digestion. However, biological solids are not recycled from output streams of fixed-film-type digesters (e.g., trickling filters and submerged fixed-film biological treatment units), nor is sludge collected in a clarifier downstream from a fixed-film-type digester recycled into the biological reactor.
The efficacy of prior art post-digestion wastewater clarification methods and apparatus varies, based on a number of factors. For instance, fixed-film biological reactors tend to have output streams containing relatively low concentrations of small solid particles that are more difficult to settle using conventional clarifiers than are solids derived from activated sludge-based biological reactors. As a result, post-clarification effluents from fixed-film biological reactors are often more turbid than desired, and further processing of the effluent can be required, adding to the duration and expense of the wastewater treatment.
A need remains for clarification apparatus and methods that can remove suspended solids more completely and efficiently than existing clarifiers, especially from wastewater streams having low concentrations of small solid particles. The invention disclosed herein satisfies this need, at least in part, by providing an improved clarifier and methods of using it.